Phase changing on-chip thermal heat sink

ABSTRACT

A method of forming an on-chip heat sink includes forming a device on a substrate. The method also includes forming a plurality of insulator layers over the device. The method further includes forming a heat sink in at least one of the plurality of insulator layers and proximate to the device. The heat sink includes a reservoir of phase change material having a melting point temperature that is less than an upper limit of a design operating temperature of the chip.

FIELD OF THE INVENTION

The invention relates to integrated circuit devices and, moreparticularly, to methods and systems for dissipating heat insemiconductor devices.

BACKGROUND

Heat can be removed from a device (e.g., transistor, power amplifier,etc.) in an integrated circuit chip using either the substrate itselfdown to a heat sink, or using wiring that is formed over the device as aheat path for transferring heat away from the device and out of the topof the chip. Such wiring, however, typically has a primary purpose ofcarrying electric current within the chip and is not primarily optimizedfor heat transfer. The electric current generates its own heat withinthe wiring through resistive heating, and the combination of resistiveheating and heat transfer from devices can degrade the reliability andthe current handling capacity of the wiring.

According to Moore's law of scaling, both the current density and thecircuit density increase with each generation. In combination withexotic substrates with limited thermal conductivity such as GaAs orsilicon-on-insulator (SOI), the thermal budget limitations in a chip arebecoming more and more severe. Circuits or subcircuits typically use thefull power budget for only a limited amount of time, often for fractionsof milliseconds. With current technology, the power and temperaturebudget need to account for the heat generated during these periods.

Accordingly, there exists a need in the art to overcome the deficienciesand limitations described hereinabove, particularly of a temporalnature.

SUMMARY

In a first aspect of the invention, there is a method of manufacturingan integrated circuit chip. The method includes forming a device on asubstrate. The method also includes forming a plurality of insulatorlayers over the device. The method further includes forming a heat sinkin at least one of the plurality of insulator layers and proximate tothe device. The heat sink comprises a reservoir of phase change materialhaving a melting point temperature that is less than an upper limit of adesign operating temperature of the chip.

In another aspect of the invention, there is a method of manufacturingan integrated circuit chip. The method includes forming a heat sink in asubstrate by: forming a trench in the substrate; forming a liner onsurfaces of the trench; forming a phase change material on the liner andin the trench, wherein the phase change material has a melting pointtemperature that is less than an upper limit of a design operatingtemperature of the chip; and forming a cap on the phase change materialand in the trench. The method also includes thinning a backside of thesubstrate to expose a portion of the liner. The method further includesforming a device on a front side of the substrate proximate the heatsink.

In another aspect of the invention, a semiconductor structure includes adevice on a substrate of an integrated circuit chip, and a heat sinkproximate to the device. The heat sink comprises a core composed of aphase change material having a melting point temperature that is lessthan an upper limit of a design operating temperature of the chip.

In another aspect of the invention, a semiconductor structure includes:a substrate; a buried insulator layer on the substrate; a semiconductorlayer on the buried insulator layer; an isolation region in thesemiconductor layer; and a resistor on the isolation region. Theresistor is composed of a phase change material that is configured to bein a liquid phase during operation of the resistor.

In another aspect of the invention, a design structure tangibly embodiedin a machine readable storage medium for designing, manufacturing, ortesting an integrated circuit is provided. The design structurecomprises the structures of the present invention. In furtherembodiments, a hardware description language (HDL) design structureencoded on a machine-readable data storage medium comprises elementsthat when processed in a computer-aided design system generates amachine-executable representation of an on-chip heat sink comprising aphase change material, which comprises the structures of the presentinvention. In still further embodiments, a method in a computer-aideddesign system is provided for generating a functional design model ofthe on-chip heat sink comprising a phase change material. The methodcomprises generating a functional representation of the structuralelements of the on-chip heat sink comprising a phase change material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIGS. 1-19 show processing steps and structures in accordance withaspects of the invention; and

FIG. 20 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test.

DETAILED DESCRIPTION

The invention relates to integrated circuit devices and, moreparticularly, to methods and systems for dissipating heat insemiconductor devices. According to aspects of the invention, a heatsink comprising a phase change material is formed in a cavity in one ormore layers of an integrated circuit chip. In embodiments, the phasechange material has a melting point temperature that is less than anupper limit of a design operating temperature of the chip. The heat sinkcomprising the phase change material may be thermally linked to anexternal heat sink that is arranged on an outer surface of the chip. Inthis manner, implementations of the invention provide an efficientmechanism for preventing temperature spikes that can be very damaging tothe components of the chip.

Aspects of the invention may be used, for example, with a sub-circuitthat uses its full power for a short duration, e.g., a power amplifierin a wireless communication system may have a full power transmissionwindow of a few microseconds. Implementations of the invention smoothout the peaks of the localized chip temperature due to such shortduration events, and this permits an external heat sink to thedimensioned less aggressively and manufactured less expensively. In anadditional application, the phase change material heat sink may be usedin it melted state as a precision resistor, relying on the fact thatmelted metal has no grains which eliminates a cause of resistorvariability.

FIGS. 1-19 show processing steps and structures in accordance withaspects of the invention. In particular, FIG. 1 shows a cross section ofa portion of a semiconductor structure 5 comprising a substrate 15, adevice 25 formed on the substrate 15, a plurality of insulator layers 35formed over the device 25 and the substrate 15, and a plurality ofelectrically conductive elements 45 formed in the insulator layers 35.The substrate 15, device 25, insulator layers 35, and conductiveelements 45 may be composed of conventional semiconductor materials andmay be formed using conventional semiconductor fabrication processes.

The substrate 15 may comprise any suitable substrate, such as asilicon-on-insulator (SOI) substrate (e.g., including a substrate, aburied insulator layer, and a semiconductor layer) or bulk materialsubstrate (e.g., including doped regions typically referred to aswells). The substrate 15 may be composed of any suitable materialincluding, but not limited to, Si, SiGe, SiGeC, SiC, GE alloys, GaAs,InAs, InP, and other III/V or II/VI compound semiconductors.

The device 25 may comprise any desired type of integrated circuit deviceincluding, but not limited to, a metal oxide semiconductor field effecttransistor (MOSFET), a heterojunction bipolar transistor (HBT), etc. Thedevice 25 may be a power device, for example, a power amplifier, a powerdiode, part of a processor core, etc., which generates a significantamount of heat during operation. The device 25 may be formed on a topsurface of the substrate 15 and may extend partially into the substrate15. Any number of devices 25 may be present in the structure 5.

The insulator layers 35 may comprise any desired number of layers ofelectrically insulating material (e.g., dielectric material), such assilicon dioxide (SiO₂), tetraethylorthosilicate (TEOS),borophosphosilicate glass (BPSG), hydrogen silsesquioxane (HSQ), etc.Such layers are commonly referred to as interlevel dielectric (ILD)layers, wiring levels, etc. A lowermost one of the insulator layers 35has a vertical thickness sufficient to cover (e.g., encapsulate) thedevice 25, while subsequent ones of the insulator layers 35 may have anydesired thickness ranging, for example, from about 0.2 μm for the lowerlayers to about 4-6 μm for the upper layers.

The plurality of electrically conductive elements 45 may comprise, forexample, contacts, wires, vias, and/or interconnects, etc., and arestructured to provide an electrically conductive pathway to a portion ofthe device 25. The electrically conductive elements 45 may be composedof any suitable material (e.g., copper, etc.) and may be formed usingconventional techniques (e.g., forming trenches in the insulator layers35 and filling the trenches with conductive material, etc.).

As depicted in FIG. 2, a trench 55 (e.g., cavity) is formed in at leastone layer of the insulator layers 35. The trench 55 may be formed usingphotolithographic masking and etching. For example, a photomask mayprovided by forming a layer of photoresist material on the uppermost oneof the insulator layers 35, exposing the photoresist material to apattern of light, and developing the exposed photoresist material. Anetching process, such as one or more reactive ion etch (RIE) processes,may then be used to form the trench 55 extending from the top surface ofthe uppermost one of the insulator layers 35 downward toward the device25 by removing material not covered by the photomask. After etching, thephotomask may be removed using a conventional ashing or strippingprocess.

Still referring to FIG. 2, the trench 55 may extend into only a singleone of the insulator layers 35, or alternatively may extend into morethan one of the insulator layers 35. In accordance with aspects of theinvention, the trench 55 is located at a distance “d” from the device 25that is: (i) sufficiently small to ensure efficient thermal couplingbetween the device 25 and a heat sink material later formed in thetrench 55, and (ii) sufficiently large to avoid inducing parasiticcapacitance between the device 25 and the heat sink material laterformed in the trench 55. In this manner, the resultant heat sink isformed proximate to the device 25. In embodiments, the distance “d” isin the range of about 5 μm to about 10 μm, although the invention is notlimited to this distance and smaller distances may be used whenparasitic capacitance can be avoided. In embodiments, the trench 55 hasa vertical depth (e.g., thickness) in a range of about 1 μm to about 5μm, and an area (e.g., in plan view) in a range of about 10 μm to about100 μm. The invention is not limited to these dimensions, however, andany suitable size trench 55 may be used within the scope of theinvention. In additional embodiments, the trench 55 vertically overlapsthe device 25 (e.g., a single vertical line intersects both the trench55 and the device 25.)

As depicted in FIG. 3, a liner 65 is formed on the exposed surfaces ofthe trench 55, e.g., contacting the material of one or more of theinsulator layers 35. The liner 65 may be formed using conventionalsemiconductor processes and may comprise any suitable diffusion barriermaterial. In embodiments, the liner 65 is formed using a conformaldeposition process, such as chemical vapor deposition (CVD), and iscomposed of a diffusion barrier material such as titanium nitride (TiN),silicon nitride (SiN), etc. The liner 65 may be formed to any desiredthickness sufficient to provide a sufficient diffusion barrier for thephase change material that will be contained in the remainder of thetrench 55. For example, the liner 65 may have a thickness in a range ofabout 4 nm to 40 nm, although other thicknesses may be used within thescope of the invention.

In embodiments, when the liner 65 is composed of an electrical insulatormaterial (e.g., SiN, etc.), the trench 55 may be formed to extend to oneor more of the electrically conductive elements 45, such that a portionof the liner 65 is formed directly on the one or more of theelectrically conductive elements 45 (e.g., an emitter contact of thedevice 25), e.g., as depicted by the dashed lines in FIG. 3. In otherembodiments, the liner 65 may be composed of an electrical conductivematerial, which permits the heat sink to be part of a wiring networkthat is electrically connected to a device or sub-circuit.

As depicted in FIG. 4, a core 75 is formed in the trench 55 on the liner65. In accordance with aspects of the invention, the core 75 comprises aphase change material having a melting point temperature that is lessthan an upper limit of a design operating temperature of the chip. In anon-limiting example, the upper limit of a design operating temperatureof the chip may be about 105° C., and the phase change material may beconfigured to have a melting point in a range between about 50° C. andabout 100° C.

In embodiments, the core 75 is composed of an alloy comprising gallium(Ga) and at least one of indium (In), zinc (Zn), tin (Sn), gold (Au),and copper (Cu). The ratio of the constituent elements of the alloyforming the core 75 may be adjusted to achieve a desired melting pointfor the core 75. The core 75 may be formed, for example, by a CVDprocess that deposits a film of the alloy (e.g., InGa) in the trenchusing a temperature controlled chamber that maintains the structure 5 ata temperature that is sufficiently low (e.g., cool) to maintain thedeposited alloy in a solid state. The core 75 may also be formed, forexample, by plural CVD processes in which layers of the individualconstituents of the alloy (e.g., In and Ga) are alternately formedwithin the trench 55. The core 75 may also be formed, for example, usingan electrodeposition process that utilizes a gallium electroplating bathwith alloying elements added directly to the bath.

Still referring to FIG. 4, the deposition of the core 75 may result inthe formation of excess material on the upper surface of the uppermostone of the insulator layers 35. The excess material (e.g., alloy) isremoved using a planarization process. Any suitable planarizationprocess may be used, such as an endpoint etch or chemical mechanicalpolish (CMP).

As depicted in FIG. 5, the core 75 is recessed to form a trench 85. Thetrench 85 may be formed using any suitable technique, such as a timedetch of the core 75 material, e.g., using an RIE process that removesthe material of the core 75 but avoids removing material of theinsulator layers 35 and liner 65.

As shown in FIG. 6, a cap 95 is formed in the trench 85 on the uppersurface of the core 75. In embodiments, the cap 85 is composed of thesame material as the liner 65. The cap 95 may be formed using anydesired fabrication technique, such as CVD. A planarization process,e.g., CMP, may be performed after forming the cap 95.

In accordance with aspects of the invention, the core 75 encapsulated bythe liner 65 and cap 95 constitutes an on-chip heat sink 100 comprisinga reservoir of phase change material that provides enhanced heatdissipation for the device 25. Heat generated by the device 25 (e.g.,when the device 25 receives a power surge) is absorbed by the on-chipheat sink 100 and causes the temperature of the core 75 to increasetoward the melting point of the core 75. As the core 75 begins to melt(e.g., change from a solid phase to a liquid phase), the heat offormation temporarily absorbs energy and keeps the core 75 temperatureclose to the melting point until substantially all the core 75 ismelted. While melting from a solid to a liquid, the core 75 remains atsubstantially a same temperature while it continues to absorb heat fromthe device 25 (e.g., as latent heat). Melting the core 75 absorbs about125 to 300 times the amount of energy required to increase a same volumeof silicon one degree Celsius. For example, heating 1000 μm³ of siliconrequires about 1.6 nJ (nano-Joule), whereas melting 1000 μm³ of InGarequires about 210-470 nJ. In this manner, implementations of theinvention keep the device 25 relatively cool at least until the core 75is fully melted.

As depicted in FIG. 7, an external heat sink 105 may be thermallyconnected to the on-chip heat sink 100. The external heat sink 105 maycomprise any conventional heat sink apparatus that is formed orconnected externally to the chip. For example, the external heat sink105 may comprise a metal layer deposited and patterned on the outersurface of the uppermost one of the insulator layers 35. As anotherexample, the external heat sink 105 may comprise a pre-formed metalstructure that is connected to the uppermost one of the insulator layers35, e.g., via a thermal interface material. The external heat sink 105promotes heat transfer away from the on-chip heat sink 100, thuspermitting the core 75 to cool and solidify when the device 25temporarily stops generating heat (e.g., between power surges of thedevice 25).

As depicted in FIG. 8, at least one insulator layer 115 may be formedover the uppermost one of the insulator layers 35 and the on-chip heatsink 100 in the structure of FIG. 6. For example, the uppermost one ofthe insulator layers 35 in which the trench 55 was formed may comprisean intermediate wiring level, and the at least one insulator layer 115may comprise a last wiring level. The at least one insulator layer 115may be composed of the same material as the insulator layers 35.

FIG. 9 shows adding an external heat sink 105 to the structure of FIG.8. In embodiments, the external heat sink 105 is formed on or connectedto the uppermost surface of the at least one insulator layer 115 (e.g.,in a manner similar to that described with respect to FIG. 7). Prior toadding the external heat sink 105, at least one thermal link 125 may beformed in the at least one insulator layer 115 and in contact with theheat sink 100. In embodiments, the at least one thermal link 125comprises a wire or via that is formed by etching a trench in the atleast one insulator layer 115 and forming a thermally conductivematerial (e.g., metal) in the trench (e.g., via CVD).

FIGS. 10-15 illustrate an implementation of the invention in which anon-chip heat sink is formed in a substrate a rather than in insulatorlayers above the substrate. In accordance with aspects of the invention,the heat sink a may be formed in a substrate as a through-silicon-via(also referred to as a through-wafer-via). For example, as shown in FIG.10, a trench 155 may be formed in the substrate 15. As described herein,the substrate 15 may be an SOI substrate or a bulk silicon substrate,and the trench 155 may be formed using photolithographic masking andetching techniques. As shown in FIG. 11, a liner 165 may be formed onthe surfaces of the trench 155. The liner 165 may be similar to liner65. As shown in FIG. 12, a core 175 may formed in the remainder of thetrench 155 on the liner 165. The core 175 may be similar to core 75. Asshown in FIG. 13, the core 175 may be recessed and a cap 195 may beformed on the core 175 to form the on-chip heat sink 100′. The cap 195may be similar to cap 95.

As shown in FIG. 14, a backside 197 of the substrate may be thinned(e.g., using a grinding process) until a portion of the liner 165 isexposed at the backside 197, such that the combination of the core 175and liner 165 constitutes a through-silicon-via 200 that extendscompletely through the substrate 15. As shown in FIG. 15, the device 25,insulating layers 35, and electrically conductive elements 45 may beformed at the front side of the substrate 15 (e.g., opposite thebackside 197) and over the on-chip heat sink 100′.

In accordance with aspects of the invention, the on-chip heat sink 100′may thus be implemented earlier in the fabrication process as athrough-silicon-via. The heat sink 100′ may undergo a phase change(e.g., melt) while absorbing heat from the device 25 (e.g., similar toheat sink 100), and may dissipate the heat through the substrate 15 oran external heat sink that is thermally linked to the heat sink 100′,e.g., at the backside 197.

FIGS. 16-19 illustrate an implementation of the invention in which aprecision resistor is composed of a phase change material. Specifically,FIG. 16 shows an exemplary SOI wafer 210 employed as an intermediatestructure in implementations of the invention. The SOI wafer 210 has asemiconductor substrate 215, which is typically a silicon substrate, aburied insulator layer 220 formed on the substrate 215, and asemiconductor layer 225, which is typically a silicon layer, formed onthe buried insulator layer 220. The constituent materials of the SOIwafer 210 may be selected based on the desired end use application ofthe semiconductor device. For example, the substrate 215 may be composedof any suitable material including, but not limited to, Si, SiGe, SiGeC,SiC, GE alloys, GaAs, InAs, InP, and other III/V or II/VI compoundsemiconductors. The buried insulator layer 220 may be composed of oxide,such as SiO₂, and may be referred to as a buried oxide (BOX) layer 220.Moreover, although the SOI wafer is referred to as “silicon oninsulator,” the semiconductor layer 225 is not limited to silicon.Instead, the semiconductor layer 225 may be comprised of varioussemiconductor materials, such as, for example, Si, SiGe, SiC, SiGeC,etc.

As shown in FIG. 17, a shallow trench isolation (STI) structure 230 isformed in the wafer 210, and a resistor 235 is formed on the STI 230.The STI 230 may be a conventional shallow trench isolation structureformed using conventional semiconductor fabrication processes andmaterials. For example, the STI 230 may be formed by arranging aphotoresist material on the semiconductor layer 225, exposing anddeveloping the photoresist, etching an STI trench in the semiconductorlayer 225 through the patterned photoresist (e.g., using an RIEprocess), stripping the photoresist, filling the trench with an STImaterial (e.g., SiO₂), and planarizing the top surface of the structure(e.g., via CMP). The STI 230 locally replaces a portion of thesemiconductor layer 225.

In accordance with aspects of the invention, the resistor 235 iscomposed of material that has a melting point that causes the resistor235 to be in a liquid phase at normal operating temperatures of thechip. For example, similar to core 75 described herein, the resistor 235may be composed of an alloy of Ga and one of and at least one of indium(In), zinc (Zn), tin (Sn), gold (Au), and copper (Cu), in which theratio of the constituent materials of the alloy is adjusted to achieve adesired melting point for the resistor 235. In embodiments, the ratio ofthe constituent materials of the alloy is adjusted to cause the resistorto have a melting point in a range of about 40° C. to about 80° C.,although the invention is not limited to these values and any suitablemelting point may be used. The material of the resistor 235 may beformed using conventional techniques, e.g., CVD, electrodeposition, etc.For example, the material of the resistor 235 may be deposited in aconformal blanket deposition and then patterned to a final shape. Asanother example, a patterned lift-off mask may first be formed, thematerial of the resistor 235 formed in an opening of the lift-off mask,and the lift-off mask removed leaving the resistor 235.

One source of variability in resistive metal films is the grain size ofthe solid metal. A liquid metal film, however, has no grains. Therefore,by using a resistor 235 that is in a liquid phase at normal (e.g.,design) operating temperatures of the chip, implementations of theinvention eliminate the unwanted variation associated with grain size.

As shown in FIG. 18, a dielectric layer 270 is formed over the resistor235, portions of the semiconductor layer 225, and STI 230. Thedielectric layer 270 may be formed using conventional semiconductorfabrication processes and materials. For example, the dielectric layer270 may comprise one or more layers of oxide, nitride, oxynitride, orother dielectric materials that are formed using, e.g., CVD. Inembodiments, the dielectric layer 270 comprises a thin oxide film 270 aformed on the resistor 235 and portions of the semiconductor layer 225and STI 230, a nitride layer 270 b deposited on the oxide film 270 a,and an upper layer 270 c (e.g., comprising SiO₂, BPSG, TESO, HSQ, etc.)deposited on the nitride layer 270 b. The oxide film 270 a may have athickness of about 3 nm, the nitride layer 270 b may have a thickness ofabout 20-30 nm, and the upper layer 270 c may have a thickness of about1-6 μm, although the invention is not limited to these dimensions andany suitable thicknesses may be employed within the scope of theinvention.

As shown in FIG. 19, resistor contacts 295 are formed in the dielectriclayer 270 and in contact with the upper surface of the resistor 235. Theresistor contacts 295 may be composed of any suitable electricalconducting material and may be formed using conventional semiconductorprocessing techniques. For example, the resistor contacts 295 may beformed by masking and etching the dielectric layer 270 to form contactholes in the dielectric layer 270, and depositing metal (e.g., copper oraluminum) in the contact holes (e.g., via CVD).

FIG. 20 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test. FIG. 20 shows a block diagram of anexemplary design flow 900 used for example, in semiconductor IC logicdesign, simulation, test, layout, and manufacture. Design flow 900includes processes, machines and/or mechanisms for processing designstructures or devices to generate logically or otherwise functionallyequivalent representations of the design structures and/or devicesdescribed above and shown in FIGS. 1-19. The design structures processedand/or generated by design flow 900 may be encoded on machine-readabletransmission or storage media to include data and/or instructions thatwhen executed or otherwise processed on a data processing systemgenerate a logically, structurally, mechanically, or otherwisefunctionally equivalent representation of hardware components, circuits,devices, or systems. Machines include, but are not limited to, anymachine used in an IC design process, such as designing, manufacturing,or simulating a circuit, component, device, or system. For example,machines may include: lithography machines, machines and/or equipmentfor generating masks (e.g. e-beam writers), computers or equipment forsimulating design structures, any apparatus used in the manufacturing ortest process, or any machines for programming functionally equivalentrepresentations of the design structures into any medium (e.g. a machinefor programming a programmable gate array).

Design flow 900 may vary depending on the type of representation beingdesigned. For example, a design flow 900 for building an applicationspecific IC (ASIC) may differ from a design flow 900 for designing astandard component or from a design flow 900 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 20 illustrates multiple such design structures including an inputdesign structure 920 that is preferably processed by a design process910. Design structure 920 may be a logical simulation design structuregenerated and processed by design process 910 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 920 may also or alternatively comprise data and/or programinstructions that when processed by design process 910, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 920 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 920 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 910 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 1-19. As such,design structure 920 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 910 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 1-19 to generate a netlist980 which may contain design structures such as design structure 920.Netlist 980 may comprise, for example, compiled or otherwise processeddata structures representing a list of wires, discrete components, logicgates, control circuits, I/O devices, models, etc. that describes theconnections to other elements and circuits in an integrated circuitdesign. Netlist 980 may be synthesized using an iterative process inwhich netlist 980 is resynthesized one or more times depending on designspecifications and parameters for the device. As with other designstructure types described herein, netlist 980 may be recorded on amachine-readable data storage medium or programmed into a programmablegate array. The medium may be a non-volatile storage medium such as amagnetic or optical disk drive, a programmable gate array, a compactflash, or other flash memory. Additionally, or in the alternative, themedium may be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the Internet, or othernetworking suitable means.

Design process 910 may include hardware and software modules forprocessing a variety of input data structure types including netlist980. Such data structure types may reside, for example, within libraryelements 930 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 940, characterization data 950, verification data 960,design rules 970, and test data files 985 which may include input testpatterns, output test results, and other testing information. Designprocess 910 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 910 withoutdeviating from the scope and spirit of the invention. Design process 910may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 910 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 920 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 990.

Design structure 990 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g. information stored in a IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 920, design structure 990 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIGS. 1-19. In one embodiment, design structure 990may comprise a compiled, executable HDL simulation model thatfunctionally simulates the devices shown in FIGS. 1-19.

Design structure 990 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 990 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIGS. 1-19. Design structure990 may then proceed to a stage 995 where, for example, design structure990: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, etc.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed:
 1. A semiconductor structure, comprising: a device on asubstrate of an integrated circuit chip; and a heat sink proximate tothe device, wherein the heat sink comprises a core composed of a phasechange material having a melting point temperature that is less than anupper limit of a design operating temperature of the chip.
 2. Thestructure of claim 1, wherein the phase change material comprises analloy comprising gallium (Ga) and at least one of indium (In), zinc(Zn), tin (Sn), gold (Au), and copper (Cu).
 3. The structure of claim 2,wherein the alloy has a melting point temperature in a range of 50° C.to 100° C.
 4. The structure of claim 1, wherein the heat sink comprisesa liner that encapsulates the core.
 5. The structure of claim 4,wherein: the liner comprises a dielectric material; the device comprisespower transistor; and an emitter contact of the power transistor extendsvertically out of the power transistor and contacts the liner.
 6. Thestructure of claim 1, wherein the heat sink is completely contained inone or more insulator layers over the device, and further comprising: anupper insulator layer formed on the one or more insulator layers; anexternal heat sink on the upper insulator layer; and at least onethermal link formed in the upper insulator layer and contacting the heatsink and the external heat sink.
 7. The structure of claim 6, whereinthe at least one thermal link comprises plural thermal links arrangedvertically and parallel to one another in the upper insulator layerbetween the heat sink and the external heat sink.
 8. The structure ofclaim 1, wherein the heat sink is completely contained in one or moreinsulator layers over the device, and further comprising: an upperinsulator layer formed on the one or more insulator layers; and anexternal heat sink on the upper insulator layer, wherein the externalheat sink directly contacts a liner that encapsulates the core.
 9. Thestructure of claim 1, wherein the heat sink is spaced apart from thedevice by a distance of about 5 to 10 μm.
 10. The structure of claim 1,wherein the heat sink vertically overlaps the device.
 11. The structureof claim 1, wherein the heat sink comprises a through-silicon-via thatextends completely through the substrate.
 12. The structure of claim 1,wherein the design operating temperature of the chip is about 105° C.,and the melting point temperature of the phase change material is in arange between about 50° C. and about 100° C.